Mechanical Anisotropy of Semicrystalline Polymer in Relation to Molecular Orientation

Abstract

A brief review on the mechanical anisotropy of crystalline polymers in bulk in relation to the molecular orientations, is presented. First, a mathematical representation of orientation distribution of structural units within the bulk polymer, is given in terms of an expansion of the distribution function in a series of spherical harmonics. Each coefficient of the expanded series is discussed in relation to several kinds of orientation factors, average degrees of the orientation distribution, defined by different authors independently. Second, several optical techniques to evaluate the orientation factors, the second and fourth moments of the orientation distribution of crystalline and noncrystalline structural units from the optical dichroic quantities, are discussed. Some experimental results on the orientation behavior of crystalline polymers during uniaxially or biaxially stretching are demonstrated. Finally, the mechanical anisotropies of oriented crystalline polymers, including uniaxial- and orthogonally biaxial-symmetric systems, are interpreted in terms of the average degrees of orientation distributions of the two kinds of structural units, crystallites and noncrystalline chain segments, and the mechanical anisotropies of the structural units for the glassy state of the polymers ; i.e., in terms of the aggregate model of biphase structure on the basis of either the homogeneous stress or strain hypothesis. Furthermore, a modification of the Krigbaum's treatment for the elasticity of the crystalline polymers, which based on the kinetic theory of entropy elasticity, to the anisotropic body, is proposed for the rubbery (or leathery) state of the polymers.